1. Field of the Invention
This invention relates to an encoding method and apparatus for encoding an input speech signal as the bitrate in the unvoiced interval is varied from that in the voiced interval. This invention also relates to a method and apparatus for decoding encoded data encoded in and transmitted from the encoding method and apparatus, and to a program furnishing medium for executing the encoding method and the decoding method by software-related technique.
2. Description of Related Art
Recently, in the field of communication in need of a transmission path, it is being contemplated, with a view to realizing efficient utilization of a transmission band, to vary the encoding rate of the input signal to be transmitted, depending on the sort of the input signal, such as speech signal interval classed into e.g., the voiced sound and the unvoiced sound, or the background noise interval, before transmitting the input signal.
For example, if a given interval is verified to be a background noise interval, it has been contemplated not to send the encoded parameters but to simply mute the interval, without the decoding device generating particularly the background noise.
This however renders the call unnatural since the background noise is superposed on the speech uttered by a counterpart of communication and, in the absence of the speech, a silent state suddenly is produced.
In this consideration, the conventional practice has been such that, if a given interval is verified to be a background noise interval, several encoded parameters are not sent, with the decoding device then generating the background noise by repeatedly employing past parameters.
However, if past parameters are consistently used in a repeated fashion, an impression is imparted that the noise itself has a pitch, so that an unnatural noise is generated. This occurs even if the level etc is changed, as long as the line spectrum pair (LSP) parameters remain the same.
It is therefore an object of the present invention to provide a speech encoding method and apparatus, input signal discriminating method, speech decoding method and apparatus, and a program furnishing medium, in which, in speech codec, a relatively large number of transmission bits is imparted to the voiced speech crucial in the speech interval, with the number of bits being decreased in the sequence of the unvoiced speech and the background noise to suppress the total number of transmission bits and to reduce the average amount of transmission bits.
In one aspect, the present invention provides a speech encoding apparatus for effecting encoding at a variable rate between voiced and unvoiced intervals of an input speech signal, including input signal verifying means for dividing the input speech signal in a pre-set unit on the time axis and for verifying whether the unvoiced interval is a background noise interval or a speech interval based on time changes of the signal level and the spectral envelope in the pre-set unit, wherein allocation of encoding bits is differentiated between parameters of the background noise interval, parameters of the speech interval and parameters of the voiced interval.
In another aspect, the present invention provides a speech encoding method for effecting encoding at a variable rate between voiced and unvoiced intervals of an input speech signal, including an input signal verifying step for dividing the input speech signal in a pre-set unit on the time axis and for verifying whether the unvoiced interval is a background noise: interval or a speech interval based on time changes of the signal level and the spectral envelope in the pre-set unit, wherein allocation of encoding bits is differentiated between parameters of the background noise interval, parameters of the speech interval and parameters of the voiced.
In still another aspect, the present invention provides a method for verifying an input signal including a step for dividing the input speech signal in a pre-set unit and for finding time changes of the signal level in the pre-set unit, a step for finding time changes of the spectral envelope in the unit, and a step for verifying a possible presence of background noise based on the time changes of the signal level and the spectral envelope.
In still another aspect, the present invention provides a decoding apparatus for decoding encoded bits with different bit allocation to parameters of an unvoiced interval and parameters of a voiced interval, including verifying means for verifying whether an interval in said encoded bits is a speech interval or a background noise interval and decoding means for decoding the encoded bits at the background noise interval by using LPC (Linear Prediction Coding) coefficients received at present or at present and in the past, CELP (Code Excitation Linear Prediction) gain indexes received at present or at present and in the past and CELP shape indexes generated internally at random if the information indicating the background noise interval is taken out by said verifying means.
In still another aspect, the present invention provides a decoding method for decoding encoded bits with different bit allocation to parameters of an unvoiced interval and parameters of a voiced interval, including a verifying step for verifying whether an interval in said encoded bits is a speech interval or a background noise interval, and a decoding step for decoding the encoded bits at the background noise interval using LPC coefficients received at present or at present and in the past, CELP gain indexes received at present or at present and in the past and CELP shape indexes generated internally at random.
In still another aspect, the present invention provides a medium for furnishing a speech encoding program for performing encoding at a variable rate between voiced and unvoiced intervals of an input speech signal, wherein the program includes an input signal verifying step for dividing the input speech signal in a pre-set unit on the time axis and for verifying whether the unvoiced interval is a background noise interval or a speech interval based on time changes of the signal level and spectral envelopes in the pre-set unit. The allocation of encoding bits is differentiated between parameters of the background noise interval, parameters of the speech interval and parameters of the voiced interval.
In yet another aspect, the present invention provides a medium for furnishing a speech decoding program for decoding transmitted bits encoded with different bit allocation to parameters of an unvoiced interval and parameters of a voiced interval, wherein the program includes a verifying step for verifying weather an interval in the encoded bits a speech interval or a background noise interval, and a decoding step for decoding the encoded bits at the background noise interval by using LPC coefficients received at present or at present and in the past, CELP gain indexes received at present or at present and in the past and CELP shape indexes generated internally at random.
With the decoding method and apparatus according to the present invention, it is possible to maintain continuity of speech signals to decode high-quality speech.
Moreover, with the program furnishing medium according to the present invention, it is possible for a computer system to maintain continuity of speech signals to decode high-quality speech.